The present invention relates to the formatting of predictive-encoded digital video signals for storage on a recording medium, especially but not exclusively an optical or magnetic disc, as well as to storage media carrying the particularly formatted signals, and encoding and decoding apparatus supporting the particular signal format.
With reference to predictive encoding, we are principally concerned with the MPEG coding schemes as defined in ISO 11172 (MPEG-1) and ISO 13818 (MPEG-2), although it will be recognised that it is applicable to other coding schemes, particularly motion-compensated and hybrid-DCT (discrete cosine transformation) based coding schemes, such as the H.261 and H.263 standards for video-telephony. MPEG and H.261 are digital coding systems conventionally used for storing and compressing natural picture sequences, with the decoders being designed to interpret such encoded data and reproduce the same original sequence of images.
The MPEG standard recognises three types of pictures (frames) namely intra-pictures, predicted pictures and interpolated pictures, generally referred to as I-, P-, and B-pictures respectively. A typical MPEG picture sequence is shown in FIG. 1 of the accompanying drawings. I-pictures are specifically intended to provide access points for random pictures and as such they are coded complete, with only limited compression. P-pictures are coded with reference to a past (I or P) picture and often form a reference for future P-pictures. B-pictures have the highest degree of compression but require both a past and future reference for their generation.
MPEG compressed video stored on an optical or magnetic disc (CD, DVD, DVD-RAM, hard-disk etc.) or other linearly-read digital storage means may be seen as a replacement for analog video storage systems such as video tape and laser disc. One of the known problems with using predictively coded video is how to implement the so-called trick modes that users take for granted (fast-forward and reverse play being the best known examples). As it is not generally possible to speed up or reverse the conventional display process, it becomes necessary to skip through the encoded data, at the same time seeking suitable access points: in the FIG. 1 example, it is only possible to start decompressing the video data stream at the points indicated, where the I-frames do not rely on contributions from prior to the access point.
The step of locating these access points brings another problem, in that the compressed frames in the sequence are of varying lengths and hence the procedure involves more than simply counting bits from the start of a previous frame. Some systems solve the problem by introducing data into the video stream with pointers to future access points (as I-frames are indicated in DVD-Video). One example of such a system is described in GB-A-2308264 where an MPEG disk for high-speed reproduction contains position information for successive I-frames in a recording area of a first I-frame. The problem with this approach is that a second pass is needed to fill in these pointers. This makes real-time recording (one pass as with conventional VCR) impossible.